My invention pertains to a fail-safe digital comparator. More specifically, the invention relates to a digital comparator arrangement for making a vital comparison between a digital word supplied broadside from one digital device and a complementary word supplied broadside from a second digital device.
There are many occasions when a pair of digital words must be compared to determine or check the correct transmission and/or reception of one or both words. Each of these words consists of a plurality of bits and is stored in one of two register devices with equal numbers of stages, that is, register devices in which each word may be stored in parallel or broadside format. A comparison can then be made by a device having a plurality of cells equal to the number of bits in a word, arranged so that the cells are appropriately controlled broadside by the two storage registers. Passage of a signal through all the cells serially is then a test that the two words are identical. In nonvital applications, each cell, for example, may be an exclusive OR element which receives an input from the corresponding bit storage stage in each word register. In this conventional apparatus, the outputs from all the exclusive OR elements are combined in a NOR circuit which supplies a final output indicating the comparison or noncomparison of the two words. The correct output occurs when the first word is identical to the second word. However, such arrangements are not vital, that is, are not fail-safe. For example, a pair of corresponding bits may both be zero and thus neither delivers any voltage to the comparator cell. This case is indistinguishable from the fault situation when one or both bit wires become disconnected from the comparator. A remedy to assure fail-safeness is to compare a first word with a complementary second word. Now a valid comparison requires that current shall flow from a bit output of one register, through the comparator cell, and into the bit output of the other register. If the comparator uses this current flow as its power supply, then a valid output indication guarantees that the wiring is intact.
Accordingly, an object of my invention is a vital digital comparator apparatus for checking the equivalence of two digital words.
Another object of the invention is vital digital comparator apparatus for comparing a digital word with its complementary digital word.
A further object of my invention is a digital comparator comprising a plurality of active cells coupled in series by optical isolator devices, which produces an output signal at the final cell, in response to an input test signal of selected characteristics applied at the initial cell, only when an actuating input is applied to each cell indicating the complementary characteristics of a pair of digital signals.
Another object of the invention is a cell element for a vital digital comparator which is coupled to the preceding and succeeding cells in a series chain by optical isolator devices and is responsive to pass a test signal between the adjacent cells only when an applied actuating signal indicates that a pair of associated digits being compared have complementary characteristics.
A still further object of the invention is a vital digital comparator, for comparing two digital words, which includes a plurality of comparator cells, one for each word digit and coupled in series by optical isolator devices, each cell receiving a rectified input when the two corresponding word digits are complementary and responsive only to such input to pass a selected signal applied to the initial cell of the comparator series.
Also an object of my invention is a vital digital comparator including a plurality of comparator cells, one for each corresponding pair of digits of two words being compared and coupled in series by optical isolator elements, each cell responsive only to a rectified input signal indicating the complementary relationship of the corresponding pair of word digits, to successively pass a test signal applied at the series input to a final output to indicate the equivalence of the two registered words.
Yet another object of my invention is a vital arrangement for comparing the equivalence of two digital words, including a storage means for each word with a storage element for each digit and a vital comparator means including a cell for each corresponding pair of word digits, these cells coupled in series by optical elements and each responsive only to an actuating input indicating a complementary relationship between the corresponding digits for passing a selected test signal from the preceding cell to the succeeding cell, the vital comparator providing an output indicating word equivalence only when each cell is conditioned to pass the selected test signal.
Other objects, features, and advantages of my invention will be apparent from the following specification and appended claims when taken in connection with the accompanying drawings.